FIELD OF THE INVENTION
The invention relates to a continuous-flow steam generator including a combustion chamber of rectangular cross-section with combustion-chamber walls each having essentially vertically disposed evaporator tubes which are connected to one another in a gas-tight manner by tube webs and through which a flow medium can flow from the bottom towards the top.
In contrast to a natural-circulation steam generator with only partial evaporation of the circulated water-water/steam-mixture, in a continuous-flow steam generator the heating of evaporator tubes forming the combustion-chamber walls leads to a complete evaporation of the flow medium in the evaporator tubes in one passage. Whereas, in a natural-circulation steam generator, the evaporator tubes are disposed basically vertically, the evaporator tubes of the continuous-flow or forced flow steam generator can be disposed both vertically and helical, and therefore at an inclination.
A continuous-flow steam generator having combustion-chamber walls which are constructed from vertically disposed evaporator tubes is more cost-effective to produce than a continuous-flow steam generator having helical tubing. Moreover, continuous-flow steam generators with vertical tubing have lower water-side/steam-side pressure losses than those with inclined evaporator tubes. However, the unavoidable differences in the supply of heat to the individual vertically disposed evaporator tubes can lead to temperature differences between adjacent evaporator tubes, particularly at the outlet of the evaporator.
Since the magnitude of the heat flow and therefore the introduction of heat into an individual evaporator tube depend on its position in the combustion-chamber wall, in the case of a combustion chamber having vertical tubing, an evaporator tube in a corner of the rectangular combustion chamber or combustion-chamber containment experiences a lower gas-side heat-flow density over its entire length than an evaporator tube in the middle of a combustion-chamber wall. The reason for that is that a flame body occurring within the combustion chamber during the combustion of a fossil fuel does not fill the en tire available space uniformly. Thus, there arises within the combustion chamber a temperature profile which is approximately bell-shaped both in the vertical and in the horizontal direction and which, starting from the middle region of the combustion chamber, decreases both upwards and downwards and towards the corners of the combustion chamber. That results in an increased supply of heat into the evaporator tubes in the middle of the combustion-chamber walls in comparison with the evaporator tubes in the region of the corners of the combustion chamber. That in turn hinders the water-side/steam-side cooling of the evaporator tubes in the middle region of the combustion-chamber walls. That can lead to inadmissibly high steam temperatures at the outlet of the evaporator tubes. Additionally, as a result of the high heat-flow density, the temperature of the tube webs in the middle of the combustion-chamber walls can assume inadmissibly high values.
Inadmissibly high temperature differences between adjacent tubes can be avoided in the vertical direction of the combustion chamber by a drastic reduction in the pressure loss attributable to friction. The reduction itself is achieved through the use of a corresponding lowering of the flow velocity or of the mass flow density in the evaporator tubes. For that purpose, however, it is necessary to use internally ribbed evaporator tubes, since they have particularly good properties of heat transmission even at low mass flow densities. Evaporator tubes of that type, with ribs forming multiple threads on their inside, and their use in steam generators are known, for example, from Published European Patent Application 0 503 116 A1.
In the case of tubing of the combustion-chamber walls of a continuous-flow steam generator with internally ribbed evaporator tubes, the axial flow has a swirl superposed on it which leads to a phase separation of the heat-absorbing medium with a water film on the inner wall of the tube. As a result, the very good heat transmission in boiling can be maintained almost up to the complete evaporation of the water. However, in the pressure range of between 200 bar and 221 bar, under strong heating, inadmissibly high wall temperatures cannot always be avoided through the use of a swirl flow alone. In the vicinity of the critical pressure of around approximately 210 bar, where there is still only a small density difference between a liquid-like and steam-like medium, the wetting of the inner wall or heating surface of the tube is substantially more difficult to guarantee than in a pressure range which is below 200 bar. That is because a steam film forming between the tube wall and the liquid phase of the heat-absorbing medium impedes the heat transmission (film boiling). In that range of steam-film formation, the temperature of the tube wall rises sharply. As is described in the paper entitled "Verdamferkonzepte fur Benson-Damferzeuger" [Evaporator Concepts for Benson Steam Generators] by J. Franke, W. Kohler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 4, pages 352 to 360, above a pressure of around 210 bar even slight wall overheating is sufficient to pass from the boiling-state with a wetted heating surface to film boiling with a non-wetted heating surface. Additionally, in the pressure range mentioned, even when slight overheating occurs, steam bubbles can form in the overheated boundary layer, and they combine to form large bubbles and thus impede the heat transmission (homogeneous nucleation).
The result of the heat transmission mechanism described is that, in the tubes of continuous-flow steam generators which are operated at pressures of approximately 200 bar and above, the mass flow density, and therefore the pressure loss attributable to friction, must be selected to be higher than in continuous-flow steam generators, which are operated at pressures of below 200 bar. That does away with the advantage that, in the case of extra heating of individual tubes, their through-put also rises. However, since high steam pressures of more than 200 bar are required in order to achieve high thermal efficiencies and therefore low emissions of carbon dioxide, it is necessary, in that pressure range too, to ensure good heat transmission. Continuous-flow steam generators with combustion-chamber walls having vertical tubing are therefore conventionally operated with relatively high mass flow densities in the evaporator tubes, in order to always achieve a sufficiently high heat transmission from the evaporator tube wall to the flow medium or heat-absorbing medium, in the critical pressure range of approximately 200 bar to 221 bar. However, those measures take into account primarily the temperature trend in the vertical direction of the combustion chamber.
A compensation of the temperature trend in the horizontal direction, and therefore a good heating balance, is achieved in the case of the helical tubing of the combustion chamber (spiral winding), since each evaporator tube or parallel tube runs virtually through all of the heating zones of the combustion chamber. However, in comparison with vertical tubing, due to comparatively small inlet surfaces of the evaporator tubes and therefore a comparatively small total number of evaporator tubes, the spiral winding leads to higher velocities of the flow medium in the evaporator tubes. That leads, in turn, to a comparatively high water-side/steam-side pressure loss.